The Numerical Calculation Of Falling Film Evaporation Process Outside Horizontal Tube Under Air-liquid Convection Conditions

The evaporative condenser is an important part of the circulating cooling water system.It has the heat exchange advantages of air cooling,water cooling and evaporative cooling.Because of its compact structure,high heat exchange efficiency,and convenient maintenance,it is widely used in metallurgy,power generation,refrigeration and other industries.The physical process outside the tube of the evaporative condenser can be summarized as air-liquid convection falling film evaporation outside the horizontal tube.At present,there are a few numerical simulation studies on the heat and mass transfer in the air-liquid convection falling film evaporation process outside the horizontal tube.The mass and energy transfer in the air-liquid convection process outside the horizontal tube is more complicated.and the falling film evaporation process can also affect the heat and mass transfer in the air-liquid convection process.Therefore,it is necessary to deeply study the local heat transfer characteristics in the falling film flow process and the heat mass transfer characteristics in the air-liquid convection process.In this paper,a numerical simulation method was used to construct a three-dimensional mathematical and physical model based on the VOF model.The research was carried out from two aspects.One aspect was the falling film flow and heat transfer outside the horizontal tube.And the other aspect was heat and mass transfer in the air-liquid convection process.Under air-liquid convection conditions,the distribution characteristics of the liquid film thickness of the horizontal tube was studied.The influence of spray density and air face velocity on the distribution of the liquid film thickness was analyzed.The results showed that in the circumferential direction,the thickness of the liquid film first decreased and then slowly increased.The thinnest position of the liquid film thickness was between 90°-100°.In the axial direction of the horizontal tube,the liquid film presented a "stabe-wave crest-stable" distribution characteristic.As the spray density increased,the thickness of the liquid film gradually increased.As the air face velocity increased,the thickness of the liquid film in the lower half of the horizontal tube increased slightly.Under air-liquid convection conditions,the distribution characteristics of the local convective heat transfer coefficient of the tube wall-liquid film outside the horizontal tube was studied.The effect of spray density and air face velocity on the local convection was analyzed.The results showed that as the circumferential angle increased,the local convective heat transfer coefficient had different distribution characteristics at different circumferential angles,which was mainly divided into "stagnation zone"(0°-6° range)and "impingement zone"(6°-30°range),"thermal layer development zone"(30°-160° range),"departure zone"(160°-180° range).In the axial direction,the local convective heat transfer coefficient decreased first,and then increased slightly at the "wave crest" position of the liquid film.With the increase of spray density,the velocity of the liquid film outside the horizontal tube increased,and the local convective heat transfer coefficient increased significantly.As the air face velocity increased,the local convective heat transfer coefficient in the middle and the lower half of the horizontal tube increased slightly.The heat transfer in the process of air-liquid convection outside the horizontal tube was studied.The influence of spray density and air face velocity on the heat and mass transfer in the air-liquid convection process was analyzed.The results showed that the proportion of latent heat transfer in the total heat transfer was about 0.75.With the increase of the spray density,the latent heat transfer had the maximum value.And the increase of the air face velocity can significantly improve the heat and mass transfer effect of the air-liquid convection process outside the horizontal tube.The effect of seawater salinity on the heat and mass transfer in the process of air-liquid convective falling film evaporation was studied.Salinity not only affects the thermophysical properties of seawater,but also directly affects the saturated vapor pressure of seawater,which affects the air-liquid water vapor partial pressure difference.The results showed that with the increased of seawater salinity,the latent heat transfer decreased in air-liquid convection falling film evaporation process.